The Preparation And Separation Performance Of MFI-type Membranes

The shortage of fresh water has already become a serious global problem due to the development of economy and the dramatic increase of population and water pollution.To solve this problem,it is necessary to derive fresh water from seawater or brackish water using desalination technology.Compared with multi-stage flash distillation and reverse osmosis,which are two of the most commercially important technologies,pervaporation has showed potential in desalination due to the feature of energy efficiency as a new type of membrane separation technology.MFI zeolite membranes are expected to exhibit high water flux and ion rejection during desalination process due to unique pore structure with⁰.5nm pore sizes and excellent hydrothermal stability.In this work,the preparation and separation performance of MFI zeolite membranes including silicalite-1 membrane（Si/Al=∞）and ZSM-5 membrane with low Si/Al ratio were studied and used in desalination of salt solution and dehydration of organic mixture based on the development of high quality desalination membrane.The main research contents and results are as following:The well-intergrown silicalite zeolite membranes were successfully synthesized on seeded aluminum oxide support in ultradilute solution with H₂O/SiO₂ ratio of 1200.The synthesis parameters such as H₂O/SiO₂ ratio in the precursor solution and crystallization time,which could seriously influence the crystals growth and membrane separation properties,were investigated.Under optimum synthe-sis conditions,silicalite membrane with high flux and ion rejection was obtained for pervaporation desalination.A better continuous MFI thin film was prepared with added ammonium salts（NH₄）₂SO₄ as crystallization mediating agents in the secondary growth.In pervaporation with 3.5 wt.%seawater feed at 75°C,silicalite-1membranes obtained showed a flux of 6.98 kg·m^-2·h^-1 and rejection of exceeding 99.96%.Hierarchically structural ZSM-5 powders and ball milled ones were used to modify the coarse support sequently to form a thin and smooth seed layer on the surface,followed by secondary growth to obtain ZSM-5 membrane of high quality.The synthesis parameters such as synthesis temperature,Si/Al ratio in the starting gel and crystallization time were investigated.High performance fibrous ZSM-5 membrane was prepared under 175°C for 48h,showing high flux of 2.00 kg·m^-2·h^-1 with corresponding separation factors higher than 10000 for dehydration of 50 wt.%AA/H₂O,and flux of 8.35 kg·m^-2·h^-1 and rejection of exceeding 99.98%for desalination of 3.5 wt.%NaCl solution at 75°C.Compared with ball milled ZSM-5 particles,silicalite-1 nano-sized crystals as small seed may lead to ZSM-5 membrane with different morphology of larger crystal sizes on the surface.ZSM-5 membranes obtained showed lower separation factor in acetic acid dehydration due to more intercrystalline defects and flux of⁷.9 kg·m^-2·h^-1 and rejection of around 99.8%for desalination of 3.5wt.%NaCl solution at 75°C.Furthermore,the degree of dissolution of the silicon source in the system could influence the nucleation crystallization process,as evidenced by the phenomenon of crystal transformation resulting from smaller silica particles as silica source.During desalination,the flux dropped from 9.28 kg·m^-2·h^-1 to 8.38 kg·m^-2·h^-1 with salt concentration increasing from 0.2 mol/L to 1 mol/L,and ion rejection remained exceeding99.97%.Based on the Arrhenius equation,the apparent activation energy(E_act)of water permeation in the ZSM-5 membrane is about 33.44 kJ/mol.After a long run test of 60 h,ZSM-5 exhibited a flux of 7.94 kg·m^-2·h^-1 and rejection of over 99.99%,showing high stability in desalination.Meantime,ZSM-5 membrane showed high separation performance and stability for dehydration of 90 wt.%AA/H₂O,95 wt.%EAC/H₂O,90 wt.%IPA/H₂O.